1. Field of the Invention
The present invention relates to a structure of a rotor for improving torque ripple reduction of a reluctance type synchronous motor used for feed drive and positioning of a machining spindle of a machine tool. More specifically, the present invention aims to improve the characteristics by adjusting magnetic reluctance change of both a rotor and a stator relative to a rotation angle of the motor.
2. Description of the Related Art
Motor torque ripples are mainly caused by permeance changes due to changes in relative positions of a rotor and a stator. By this permeance change, magnetic energy stored in void spaces between a rotor and a stator changes, which leads to torque ripples. More specifically, stator slot ripples due to stator tooth and pole ripples due to magnetic poles of a rotor are cited as causes of torque ripples.
As a method to reduce torque ripples (mainly slot ripples) of a motor creating magnetic field using a permanent magnet and used for servo mechanism, a technique to skew a central angle of magnetic poles of a stator or a rotor in rotational direction by an angle equivalent to one or two times a pitch angle of a slot (the angle depending on winding order) has been known conventionally.
The main objective of this technique is to cancel phases of torque ripples in macroscopic view by skewing rotor or stator magnetic poles.
FIG. 14 shows an example of a conventional reluctance type synchronous motor.
For convenience in explanation, an example with 6 air slits per one magnetic pole of a rotor, 4 rotor magnetic poles rotor, and 12 stator slots is shown. Slits are not necessarily air slits, and the numbers of magnetic poles and slots are not limited to the numbers used here.
The stator 1 is composed of a layer of thin, soft magnetic material plates made of a material such as silicon steel. The stator comprises teeth and slots. Each slot stores a stator winding 5.
The rotor composed of a layer of thin, soft magnetic material plates 2 which includes magnetic isolating portions 4 creating magnetic paths in the plates 2. The magnetic isolating portions 4 can be composed of non-magnetic material such as resin or aluminum. This example uses air which is the simplest and most convenient magnetic isolating portion. In other words, the soft magnetic material 2 is filled with air stored in the slits.
In this example, the rotor structure is implemented by layering soft magnetic material plates. Therefore, minute joints exist at the periphery of the rotor (so that the rotor will not break into pieces due to the slits of the magnetic isolating portions 4). Magnetic short circuits in these joints can be ignored by thinning the joints.
If soft magnetic material plates are cut into a horse shoe shape and layered/fixed radially along a q-axis 7 and centered at a rotational shaft 3 (generally called a "flux barrier type reluctance motor"), the minute joints described above are not necessary.
The magnetic poles defined at the rotor by the magnetic isolating portions 4 have two imaginary axes used for explanation of motor control. One of these is a d-axis 6 which is a magnetic pole center and the other is the q-axis 7 which is a magnetic boundary of the neighboring poles.
For motor control, a field current (d-axis current) which generates a field component flows through the d-axis 6, while an armature current (q-axis current) which generates a torque component flows through the q-axis 7.
By the flow of the field current, a solenoid equivalent to a permanent magnet of a permanent magnet motor is created at the rotor. The armature current is exactly the same as an armature current of a permanent magnet motor. Therefore, by controlling d-axis and q-axis currents, a torque and control performance of a reluctance motor, which are equivalent to a permanent magnet motor and according to the Flemming's left hand law, can be obtained.
As is already known, small magnetic domains are created within a permanent magnet of a permanent magnet motor, and each magnetic domain has a magnetomotive force. In other words, a magnetic flux density at the surface of a magnetic pole can be almost even, since there exists means to confine the small magnetic flux.
However, in a relationship between teeth of the stator 1 and magnetic paths of a rotor of a reluctance type synchronous motor (composed of soft magnetic material 2 and the magnetic isolating portions 4), the soft magnetic material creating magnetic paths in rotational direction exists separately, which leads to dispersed magnetic energy stored in void spaces between the stator 1 and the rotor.
The dispersed magnetic energy appears as torque ripples upon controlling of the motor, and may cause troubles such as noise or undulated machining patterns of work piece when the reluctance motor of this kind is used for feeding the rotational shaft.
As described above, in a permanent magnet motor using a permanent magnet on a rotor, magnetic domains are created within the permanent magnet, and each domain has a magnetomotive force. Therefore, a magnetic flux density at the surface of magnetic poles is almost even, since there exists means to confine the magnetic flux. As a result, skewing of magnetic poles of a rotor or a stator can easily reduce torque ripples (especially slot ripples due to a stator slot period).
However, if a rotor of a synchronous motor is composed of a soft magnetic material and rotates in synchronization with rotating magnetic field of a stator, magnetic flux to passes an area of low magnetic reluctance within magnetic poles of the rotor. Therefore, reducing torque ripples by skewing is difficult due to the fact that a gap width of a rotor's magnetic poles relative to a stator's magnetic poles is different (i.e., an effective gap width of the rotor is reduced due to skewing), which leads to leakage of magnetic flux towards low magnetic reluctance areas within the rotor.